1. Field of the Invention
The present invention relates to a module for optical transmitter including an opto-electronic integrated circuit chip obtained by forming an opto-electronic optical modulator and a driver circuit for modulator as an integrated circuit, and an optical transmission system using the same.
2. Description of the Related Art
In a conventional optical transmitter, a multiplexer, an optical modulator, and a driver circuit for modulator are different modules, and the respective modules are provided on the base of the optical transmitter. Further, the connection between the respective modules is established by a characteristic impedance-matched cable or connector. The photograph of the external view thereof is disclosed in xe2x80x9cElectronic Technologyxe2x80x9d, 2000, Nov. issue, pp. 7-8.
Further, the system block diagram thereof is disclosed in xe2x80x9cFIFTH ASIA-PACIFIC CONFERENCE ON COMMUNICATIONS AND FOURTH OPTOELECTRONICS COMMUNICATIONS CONFERENCE, APCC/OECC""99, PROCEEDINGSxe2x80x9d, pp. 12-13.
In the foregoing prior art, a multiplexer module, a module of a driver circuit for optical modulator, and a module of an optical modulator are connected by a cable or a connector. Accordingly, the connection loss of an electric signal occurs at the connection part, resulting in degradation in response characteristic from the electric signal to an optical signal.
Further, the area occupied by the modules is increased due to the cable or the connector, resulting in a large transmitter.
The present inventors prototyped a module for optical transmitter wherein an optical modulator and a driver circuit for modulator are formed as an OEIC for implementing a compact high-performance optical transmitter. FIG. 2 is a top view of the interior of the prototyped module for optical transmitter seen by removing the upper part of the storage case thereof. FIG. 3 is a side view seen from the cross section of the portion along line D-Dxe2x80x3 shown in FIG. 2. FIG. 4 is a perspective view of the prototyped OEIC chip. FIG. 5 is a cross sectional view of the portion along line E-Exe2x80x2 shown in FIG. 4.
For the prototyped module for optical transmitter, as shown in FIG. 2, an OEIC chip 80 is connected to an Au (gold)-plated wiring pattern 82 formed on a ceramic substrate 81 by a bonding wire 83. A thermister 84 for detecting the inside temperature of the module is also connected thereto similarly, and connected to an input/output (I/O) terminal 86 by a lead wire 85.
A storage case 87 made of a metal serving as a housing is provided with a connector 88 for inputting a high-frequency signal to the OEIC chip 80 and a terminal 89 for applying dc voltage. Each terminal is connected to the Au-plated pattern 82 formed on the ceramic substrate 81 by each lead wire 85. A single mode fiber 90 is connected to an isolator 91, and inputs and outputs optical signals to and from the OEIC chip 80 through an aspherical lens 92 for fiber coupling.
Further, as shown in FIG. 3, the OEIC chip 80 is fixed on a carrier 93 made of CuW (copper tungsten). Further, the carrier 93 is connected to a peltier cooler 94, and placed on the bottom 95 of the case 87. Thus, the module is so configured that the OEIC chip 80 is cooled by passage of a prescribed current through the I/O terminal 96 of the peltier cooler 94.
Further, for the prototyped OEIC chip 80, as shown in FIGS. 4 and 5, an opto-electronic optical modulator 97 and a driver circuit 98 for modulator are formed on the same InP substrate 99. Further, a characteristic impedance-matched transmission line (G-S-G line) 100 composed of ground wire G-signal wire S-ground wire G for connecting the optical modulator 97 and the driver circuit 98, a pad 101 for input signal, and a pad 102 for dc bias are included therein. An interlayer insulating film 103 is formed under the respective pads 101 and 102, and the transmission line 100. To the opto-electronic optical modulator 97, an optical waveguide 104 is connected. Further, the substrate 99 made of InP has been reduced in thickness to 100 xcexcm, implementing such a configuration as to enhance the heat dissipation effect.
The present inventors mounted the foregoing prototyped module for optical transmitter in an optical transceiver system, and evaluated the characteristics. As a result, they found that the module for optical transmitter using the OEIC chip 80 is more degraded in terms of optical transmission characteristics than the one manufactured by using separate modules.
Further, the present inventors considered that this is caused by the thermal transmission from the driver circuit 98 for modulator with large power consumption to the optical modulator 97, and performed the numerical analysis on the thermal resistance for defining the thermal transmission path.
FIG. 6 shows a simplified thermal path of the prototyped module for optical transmitter. This thermal path is composed of the thermal resistance Rth1 of the semiconductor substrate 99, the thermal resistance Rth2 of the wiring metal formed on the surface of the OEIC chip 80, the thermal resistance Rth3 of the gas filled in the module for optical transmitter, the input power P1 of the OEIC chip 80, the input power P2 of the peltier cooler 94, the temperature T1 of the driver circuit 98 for modulator, the temperature T2 of the optical modulator 97 portion, and the ambient temperature Ta.
The thermal resistances Rth1 to Rth3 of the respective elements are expressed by the following equation (1).
Rthn=L/xcexAxe2x80x83xe2x80x83(1) 
wherein n=1, 2, or 3; L denotes the distance between two points of the optical modulator and the driver circuit for modulator; xcex, the thermal concuctivity; and A, the cross-sectional area.
The analysis was performed assuming as follows: the distance L is 1 mm; the thickness of the wire, 5 xcexcm; the width of the wire, 1 mm; and the width of the OEIC chip, 1.7 mm. As a result, the thermal resistance Rth2 on the substrate surface was 250xc2x0 C./W. In contrast, the thermal resistance Rth1 of the substrate portion was 33.4xc2x0 C./W, which is about 13% of the thermal resistance of the substrate surface.
The prototyped module for optical transmitter is an enclosed housing. Accordingly, the thermal transmission by a gas is considered to be smaller than the thermal transmission from the wiring metal. Thus, the thermal resistance Rth between the optical modulator 97 and the driver circuit 98 for modulator is determined two dimensionally in a simplified manner, and it can be expressed as the following equation (2):
Rth=(Rth1xc3x97Rth2)/(Rth1+Rth2)xe2x80x83xe2x80x83(2) 
The thermal resistance between the two points can be considered as a simple combined resistance. Therefore, it has been shown that the heat generated at the driver circuit 98 for modulator was conducted mainly through the substrate 99 to the optical modulator 97 to degrade the extinction characteristic of the optical modulator 97.
Further, although the peltier cooler was disposed in the prototyped module for optical transmitter, presumably, it was not capable of controlling the temperature rise and the temperature change in the optical modulator 97 due to the thermal path along the lateral direction of the substrate 99 from the driver circuit 98 for modulator.
Herein, simple provision of a large-capacity peltier cooler exhibiting a large cooling effect may also be mentioned as one of the countermeasures. This, however, leads to an increase in power consumption of the module for optical transmitter itself, and hence is not a preferred countermeasure from the viewpoint of implementing lower power consumption of the optical transmitter.
Therefore, it is essential to solve the thermal transfer problem for improving the performance than that of the conventional optical transmitter.
Under such circumstances, it is therefore an object of the present invention to provide a module for optical transmitter mounting therein an OEIC chip in which an optical modulator and a driver circuit for modulator are integrated for implementing a high-performance compact optical transmitter.
Further, it is another object of the present invention to provide an optical transmission system using the module for optical transmitter mounting the OEIC chip.
Still further, it is a still other object of the present invention to provide a technology for reducing the temperature rise in the driver circuit for modulator in the OEIC chip.
Furthermore, it is a still further object of the present invention to provide a technology for stabilizing the thermal fluctuations in the optical modulator.
The forgoing objective module for transmitter is implemented by using the following configurations (1) and (2).
(1) Cooling plates are disposed on the wiring of the OEIC chip to increase the cooling area; and
(2) Apart of the semiconductor substrate immediately under the transmission line establishing the connection between the opto-electronic optical modulator and the driver circuit for modulator is reduced in thickness, or removed. In addition, the carrier mounting the OEIC chip thereon is divided into two parts, and further, a cooling element such as a peltier cooler is mounted on the optical modulator side. Namely, with the configuration (1), the heating density is reduced to reduce the thermal resistance, so that a reduction in the temperature rise in the driver circuit for modulator is implemented. Whereas, with the configuration (2), the heat conductivity via the substrate is reduced, and the opto-electronic optical modulator is cooled with efficiency, thereby achieving the thermal stabilization thereof.
Further, for the foregoing objective optical transmission system, the module for optical transmitter using the configurations (1) and (2) is used for the optical transmitter in the optical transmission system. As a result, it is possible to implement an optical transmission system which is compact and excellent in transmission characteristic because of the reduced number of modules.
Herein, one example of typical means of the module for optical transmitter in accordance with the present invention will be shown as follows.
A module for optical transmitter of the present invention is a module for optical transmitter mounting therein an OEIC chip including an opto-electronic optical modulator and a driver circuit for modulator formed as an opto-electronic integrated circuit on the same semiconductor substrate, characterized in that the OEIC chip has first and second semiconductor substrate areas having a large thickness and separated by a groove formed by thinning a part of the semiconductor substrate immediately under a wiring metal for connecting the opto-electronic optical modulator and the driver circuit for modulator, the opto-electronic optical modulator is formed on the first semiconductor substrate area side, and the driver circuit for modulator is formed on the second semiconductor substrate area side, a first carrier for mounting the first semiconductor substrate area side of the OEIC chip, and a second carrier for mounting the second semiconductor area side of the OEIC chip are placed apart from each other, a peltier cooler is connected to the first carrier, and further a protruding cooling plate is formed on the driver circuit for modulator on the OEIC chip.